Controlled sequence pressure nip

ABSTRACT

A press structure for performing a dewatering operation in the steps of formation of a traveling fibrous web such as in a paper making machine wherein the wet web is carried on one or more water absorbing felts through a press formed of a pair of extremely tough liquid impervious belts with the belts backed throughout a pressing zone by a series of fluid pressure chambers applying hydraulic pressures to the back of the belt. The chambers are arranged so that a first fluid pressure is applied at a first portion of the pressing zone, and subsequently a second higher pressure is applied to the belts at a second portion of the pressing zone and thereafter a third pressure is applied at a third portion. Each successive pressure is higher than the previous one so that the hydraulic resistance pressure of the moisture leaving the web does not build up at such a rate so as to disrupt the web fibers.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a continuation-in-part of application Ser. No.412,578 filed Nov. 5, 1973 which in turn is a continuation ofapplication Ser. No. 241,710 filed Apr. 6, 1972, both now abandoned.

BACKGROUND OF THE INVENTION

The invention relates to improvements in presses for extracting waterfrom a continuing traveling web such as a newly formed web in a papermachine, and particularly the invention relates to a structure forproviding an extended press nip which applies a pressing force to a webfor a longer continuous time than structures of the type conventionallyused such as formed by the nip of opposed roll couples. The inventionparticularly relates to an improved structure and method for obtainingand squeezing more water from the web than heretofore possible andaccomplishing this function without disruption of the web fibers toobtain the formation of an improved web.

In the copending application of Busker and Francik, Ser. No. 193,272,now U.S. Pat. No. 3,798,121, the principles and advantages of pressing apaper web for an extended period of time and the advantages thereof arediscussed. In the present structure, the principles of an extended nipare utilized in a structure affording advantages over prior artarrangements. In prior art structures such as conventional opposedrolls, the pressing pressure applied to a web is applied suddenly as theweb passes through the nip and suddenly released to be again applied ata succeeding nip. In high speed paper machines, the pressure is appliedvery suddenly and over a very short period of time, and it has beenfound that hydraulic pressures due to flow resistance build up withinthe web preventing the water from escaping. If the pressure isincreased, the amount of water removed is not significantly increasedbecause of the resistance of the water to escaping in the relativelyshort period of time. Further disadvantages are encountered in that webfiber disruption, commonly called "crushing", occurs if the waterpressures build up too rapidly within the nip.

It is accordingly an important feature of the present invention toprovide an improved extended nip press which applies pressures stepwiseto the web so that the water within the web can flow out at an optimumrate to achieve maximum dewatering without fiber disruption.

As will be appreciated from the teachings of the disclosure, thefeatures of the invention may be employed in the dewatering of otherforms of webs than a paper web in a paper making machine. However, forconvenience, a preferred embodiment of the invention will be describedin the environment of a paper making machine which conventionally formsa web by depositing a slurry of pulp fibers on a traveling fourdrinierwire, transfers the web to a press section where the web passes througha number of press nips formed between roll couples, and the web thenpasses over a series of heated dryer drums and usually through acalender and then is wound on the roll. The present structure forms theentire press section and takes the place of other forms of presssections heretofore available. Many modifications can be made in thistype of overall machine, as to the forming section, the press section,the dryer section, and the structure of the instant disclosure may beemployed in pressing webs of various synthetic fibers.

The present invention relates to improvements for the press sections ofa paper making machine. In such a machine the web usually arrives at thepress section with about 80 percent web basis moisture (ratio of waterto fiber plus water) and leaves the press section with approximately 60percent moisture, with the remaining moisture having to be removed bythermal evaporation in the dryer section as the web passes over a seriesof heated dryer drums. Because of various inherent limitations in theoperation of roll couples forming press nips for the press section in aconventional paper making machine, only a given amount of water can beremoved in each nip and, therefore, in a conventional paper makingmachine, a series of three press nips are usually employed. It has beenfound impractical to attempt to remove a significant amount ofadditional water by increasing the number of press nips, although thefurther removal of water by pressing can greatly reduce the expense andsize of the dryer section. It is estimated that if the water removed inthe press section can be increased to decrease moisture from 60 percentto 50 percent, the length of the dryer section can be reduced byone-third. This is significant in a typical 3000 feet per minutenewsprint machine which employs on the order of 100 dryer drums. Thesignificance can be appreciated in considering that the dryer drums areeach expensive to construct and to operate and require the provision ofsteam fittings and a supply of steam for each drum. The relativeimportance of the removal of water in the press section is furtherhighlighted by the fact that one of the most important economicconsiderations in justifying a satisfactory return on investment in theoperation of a paper making machine is to obtain the highest speedpossible consistent with good paper formation and better pressing willshorten the necessary time in the dryer section and permit higherspeeds.

It is accordingly an object of the present invention to provide animprovement in the press section of a paper machine which makes itpossible to remove an increased amount of water in this press secton andmakes it possible to provide a press section having only a singlepressing nip of a unique elongated or extended nature which does nothave the performance limitations of conventional roll couple presses andwhich requires far less space in terms of requirements as to the overalllength of the press section. By increasing the amount of water removedfrom the web in the press section, increased speeds are possible withexisting equipment, i.e., a given length of dryer section can operate athigher speeds since it is required to remove less water. Also, newequipment can be constructed requiring less machine length and expense.

The present invention employs a principle which may be referred to asthe extended nip concept wherein the time the web is subjected to apressing action is greatly extended, i.e., a single pressing is providedhaving a residence time which exceeds that of the time of the web in anumber of conventional roll couple press nips. With the reduction to asingle pressing operation, the compound effects of rewetting the web asit leaves a plurality of nips are avoided.

A factor which presently limits water removal from paper by mechanicalweb pressing is the flow property of water within the paper sheet. Ithas been found that other factors are not of dominant significance, forexample, the effects of the moisture in the left which travels with theweb are small. It has been found further that the length of time thatthe web is in the nip, in other words the residence in the nip, can havea significant effect in overcoming the difficulties created by the flowproperties of the water within the sheet. It has also been found thatmerely by increasing the residence time of the web in the nip, the watercontent of the sheet coming out of the press can be decreased so thatthe web will have 46 percent dryness rather than 40 percent dryness withother variables remaining constant. As is evident, the residence time ofa web in a conventional roll couple press nip is limited and can only beincreased by decreasing the speed of travel of the web, or can beincreased slightly by increasing the diameter of the press rolls, butthese factors are indeed limiting. It has been found, for example, thatby applying a 1300 pound per square inch pressure on a web for fiveminutes, a moisture level of less than 30 percent can be attained. Yet,under the dynamic short term mechanical pressing of a paper machinepress section using roll couples, even with a plurality of nips, a greatdeal of effort is required to maintain moisture levels below 60 percent.

It has been found that significant losses in dryness occur at higherspeeds and that a loss in dryness of over 5 percent is experienced ingoing from 300 feet per minute to 1000 feet per minute with typicalpress loadings in suction press. It has been found that a hydraulicpressure or wedge effect develops during the passage of the wet matthrough the wet press nip. The hydraulic pressure that developssubstracts from the applied load and reduces the mechanical compactingpressure. The result is a loss in dryness. As the machine speedincreases, the compacting rates are higher, resulting in higherhydraulic pressures within the paper mat. These hydraulic pressuresreact against the pressure of the rolls and prevent the moisture frombeing squeezed from the web. The exact value of hydraulic pressure isdifficult to determine either by direct measure or analysis because ofthe space and speeds involved. It is believed, however, that hydraulicpressure predominately determines press performance on machinesoperating at high speeds. Accordingly, the instant invention relates toavoiding disadvantages encountered with high speed press nips of theconventional type used in most commercial applications today, andprovides a substantial increase in residence time within a press nip toallow time for flow to occur withint the mat and for the hydraulicpressure to dissipate. The principles of extended nip or extended timepressing are further reviewed in the aforementioned application.

SUMMARY OF THE INVENTION

The present invention increases the dryness of the web leaving the presssection by passing the web through separate, successive, adjacentportions of the extended pressing zone wherein successively increasinghydraulic pressure is applied to the web. This prevents water previouslyexpressed from the web from reentering as the web passes into the nextportion of the pressing zone. This invention also exploits theprinciples associated with the phenomenon of web crushing by applying acomparatively low pressure in the first portion of the extended zone,where the web moisture is highest, and comparatively high pressure inthe last portion of the zone, where the web moisture is lowest. Thus, inthe first portion where the web is more incompletely formed, the wateris comparatively gently urged out of the web so as to not disturb theposition of the water laid fibers forming the web. If water is forcedout of a web too fast, thin spots and even holes are formed in the webwhere the fibers have been displaced, thus degrading the web quality.

In succeeding portions of the zone, where the web is more completelyformed, greater pressure can safely be applied without crushing the web.Thus, the maximum pressure consistent with maximum water removal withoutweb crushing can be applied for an extended period while the web isbeing dewatered and is becoming relatively more dry. The faster the webbecomes drier, the sooner increased pressure can be applied to speed updrying. However, since the web is progressively becoming drier, the rateof water removal decreases with each successive portion of the pressingzone.

In addition, the pressure chambers in the chamber housings arepressurized with water, or other liquid, which is intended to leak outof their periphery in a controlled manner to provide lubrication betweenthe chamber housings and the tough, liquid impervious belts supportingthe web. The rate of this leakage is constant since each pressurechamber is supplied by a constant flow control.

It is accordingly an object of the present invention to provide amechanism which will enable pressing a high speed traveling web over arelatively extended period of time so as to overcome counter-hydraulicpressures and to achieve improved water removal in the press.

A further object of the invention is to provide a press of a type abovedescribed wherein the principles of an extended nip press can beutilized in an improved form to achieve improved and uniform pressureswith a mechanism capable of operating at high speeds and rapidlyextracting water without fiber disturbance (crushing) to form animproved web.

Other objects, advantages and features will become more apparent withthe disclosure of the principles of the invention, and it will beapparent that equivalent structures and methods may be employed withinthe principles and scope of the invention, in connection with thedescription of the preferred embodiment and teaching of the principlesof the invention in the specification, claims and drawings, in which:

DESCRIPTION OF THE DRAWINGS

FIG. 1 is a side elevational view, shown somewhat in schematic, of amechanism embodying the principles of the present invention.

FIG. 2 is a graph illustrating the pressure on the web as a function ofposition of the web as it passes through the pressing zone.

FIG. 3 is a perspective view of the piston-like apparatus forming eachof the chamber housings in FIG. 1.

FIG. 4 is a perspective view of the beam into which the chamber housingsare slidably mounted.

FIG. 5 is a view through section A--A of FIG. 1.

FIG. 6 is a side elevational view, shown somewhat in schematic, ofanother mechanism embodying the principles of the present invention.

FIG. 7 is a perspective view of the flexible diaphragm forming the outerwalls of the supporting chambers in FIG. 6.

FIG. 8 is a perspective view of the beam into which the flexiblediaphragm shown in FIG. 7 is mounted.

FIG. 9 is a sectional and elevational view of the diaphragm shown inFIGS. 6 and 7.

FIG. 10 is a partial view of two adjacent chamber housings, showing themanner of lubrication with water against the traveling belt.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

As shown in FIG. 1, a web W is laid onto a traveling felt F to passthrough the extended nip press shown in the drawing. The web laid on thefelt is carried between looped flexible belts 10 and 11. Each of thebelts is of a heavy extremely tough, flexible, liquid imperviousmaterial such as reinforced rubber, thin metal or plastic to carry theweb and felt therebetween and to transmit hydraulic pressure which isapplied to the outer surface of the belts to press the web sandwichedtherebetween. The pressed web is moist having come from the formingsection of a paper machine such as from a fourdrinier forming wire.Suitable pick-up mechanism is provided to transfer the web from thefourdrinier section to the press of FIG. 1, and suitable additionalequipment will be provided to receive the paper web as it leaves thepress to carry it onto a thermal drying section of the machine andsubsequently onto a calender or other equipment for completing theprocessing of the web.

The belts may each be driven such as by driving their carrying rolls,and the upper looped belt is carried on rolls such as 12 and 13positioned to guide the belt through the press. The lower looped belt isguided on similar rolls 14 and 15.

The pressing structure for applying a hydraulic pressure to the belts isshown at 16 for the top belt and at 17 for the bottom belt. During thetime the web is carried between the belts, it is exposed to thehydraulically applied pressure and this period of travel will bereferred to as the pressing zone.

The pressing zone is arranged in steps or portions, and a successivelyhigher pressure is applied at each portion of the pressing zonethroughout the extended nip press. With this method, the pressure of theliquid escaping from the web can build up to an optimum point formaximum egress of the liquid, but not to a point where excessiveresistance pressure occur within the web such as to disorient or disruptthe fibers within the web. The flow is orderly through the fibers fromthe web into the felt. Since the extended nip subjects the web to acontinuous pressure over an adequate length of time, the water canescape from the web until optimum dryness from mechanical pressing isobtained. The pressures chosen for each successive portion of thepressing zone can be determined experimentally, but are related to thevarious factors that affect the nature of the web and its moisturecontent. The hydraulic pressures which will build up will be dependentupon the type, size and length of the fibers employed, the thickness ofthe web, its initial formation, the type of web handled, the temperatureof the water and so forth. For optimum operation, the initial pressureis at least 100 pounds per square inch, and the pressure in the finalchamber will be on the order of 600 pounds per square inch or greater.As an example, with the structure shown in FIG. 1 which employs threesuccessive portions of the pressing zone, the initial chamber may have apressure of 200 pounds per square inch, the next chamber 400, and thethird 600 pounds per square inch. Inasmuch as the pressure is appliedhydraulically, uniform pressure will exist over the entire width of theweb for the length of the portion of the zone to obtain uniformdewatering.

Throughout the specification, alphabetical subscripts will be used todistinguish identical items in a figure and primes will be used todesignate similar or corresponding items in other embodiments orfigures.

The first portion of the pressing zone is furnished by the opposed beltpressure chambers 20a and 21a as shown in FIG. 1. The next portion isprovided by the opposed belt pressure chambers 20b and 21b, and the lastportion of the pressing zone is provided by opposed belt pressurechambers 20c and 21c. These pressure chambers are formed in the beltengaging ends of the corresponding piston-like chamber housings 22a and23a, 22b and 23b, and 22c and 23c, respectively. Depending upon thetypes of web handled and other factors such as the amount of water to behandled, as few as two portions may be employed or substantially morethan three may be employed if necessary. For handling the dewatering ofa conventional paper web, three chambers will provide for an adequaterelease of water without building up excess flow velocity of waterwithin the web.

Each of the portions of the pressing zone, as constituted by thepressing chamber, is of substantially the same construction and,therefore, only the structure which forms the belt pressure zone portion20a need be described in detail.

The pressure chambers are hydraulically held against the belt, and thepressure chambers for the entire pressing zone are shown as supported onopposed beams 18 and 19 which may be either of unitary construction ordivided into individual beams 18a - 18c, 19a - 19c corresponding tochamber housings 22a - 22c, 23a - 23c as shown by dash lines 50, 51, 52,53. These beams may bow upwardly with the application of pressure, butin the arrangement illustrated, each chamber housing that forms the beltpressure chamber is itself hydraulically supported so that the bendingupwardly and downwardly of beams 18 and 19, respectively, will notadversely affect the hydraulic pressure applied to the belts, but willoperate to provide uniform pressure on the belts.

The chamber 20a is provided in chamber housing 22a which has sidewallsor edges 24a, 24a', 25a, 25a' in sliding engagement with the belt 10 asshown in FIGS. 1, 5, 6 and 10. The wall engaging the belt on theoncoming side of the chamber is shown at 24a, and the wall or sillengaging the belt on the off running side of the chamber 20a as shown at24a'. Similar walls 25a, 25a', extending in the direction of belttravel, are provided at the side of the chamber so that the liquidwithin the chamber 20a is confined to the desired extent to apply itspressure to the flexible belt.

As shown in FIG. 1, chamber housings 22a, b, c are movable independentlyof one another in beam 18. A slightly modified embodiment would be todivide beams 18, 19 into smaller beam sections 18a, b, c; 19a, b, ccorresponding to chamber housings 22a, b, c; 23a, b, c, respectively.This is shown by dash lines 50, 51, 52, 53.

Chamber housing 22a is backed by liquid in a supporting chamber 26a.Similarly, chamber housing 23a is backed by liquid in supporting chamber27a. Seals 48a, b, c; 49a, b, c secure the interface between rigidstructural peripheral walls 60a, b, c; 61a, b, c of the beams and thehousing chambers against fluid escape. Fluid under pressure, preferablywater, is directed into the supporting chambers 26a, 27a throughhydraulic supply lines 29a, 71a and valves 30a, 70a which control thepressure. Additional fluid, preferably water, is introduced underpressure into pressure chambers 20a, 21a through hydraulic supply lines80a, 81a and the flow or volume quantity is controlled by valves 82a,83a. The water introduced into pressure chamber 20a may be under thesame or different pressure as the water in supporting chamber 26a, asdetermined by the respective areas of each chamber.

If it is anticipated that nearly the same pressure will be constantlyused in the supporting and pressure chambers, an alternate embodimentwould be to connect them via hydraulic conduits 84a, 85a in the chamberhousing as shown in dashed lines. In this case, the upper area of eachchamber housing would be designed to be in balance with the area of thepressure chamber bearing against the belt to insure the desired sealingforce of the chamber housing edges against the belt when some hydraulicpressure loss occurs in conduit 84a, 85a. These edges may be of metal ormaybe coated with Teflon (Reg. T.M.) or other low friction material thatprovides the degree of desired sealing and which permits the belt totravel past at the speed of travel of the paper web. The belt, and, ifdesired, the seal-like edges of the walls, are sufficiently flexible topermit flexure and degree of desired sealing as the belt travels.

In fact, the seal forming the interface between edges 24a, 24a', 25a,25a' of the pressure chamber and the traveling belt is intended to berelatively loose to permit a controlled amount of water to escape acrossthe edges from the pressure chamber to provide lubrication of thesliding surfaces. The water escaping between edges 24a', 24b travelsupwardly, as shown by the arrows in FIG. 10, where it is removed bymeans, not shown, such as a drainage conduit or a suction pump.

Another embodiment is shown in FIG. 6. Each chamber housing, such as22a', is attached to the beam by a flexible, peripheral diaphragm 90, asshown in more detail in FIGS. 7 and 9. A metal fabric 94 provides thestrength to withstand the high hydraulic pressures required. Thediaphragm itself is constructed of an elastomeric material, such asrubber. The diaphragm is secured to the beam and a chamber housing withsuitable means, such as screws and clamping bars to define the flexiblewalls of support chambers 26a', b', c' and 27a', b', c'.

In order to insure that the chamber housings are maintained in properalignment while being capable of small upward and downward movementunder operating conditions, at least one guide rod 96a (97a) is attachedto the upper side of each chamber housing 22a' (23a') and slidablyguided in beam members 18a' (19a').

The manner of pressurizing both pressure chambers 20a' and chamberhousing 22a' is the same as in the embodiment shown in FIG. 1, that isthey can be either independently pressurized through hydraulic supplylines 29a', 80a' or interconnected through a connecting hydraulicconduit 84a' to be pressurized at nearly the same pressure, as desired.

As diaphragms 90a, 90b on adjacent chamber housings are pressurizedand/or move up and down in operation, there may be some slight relativemovement or contact, but since they are flexible, this does not raiseany stress or strain. Total upward or downward movement of the chamberhousings in operation is anticipated to range from about 0.001 inches toabout 0.020 inches, depending on such factors as belt and feltconstruction and the nature of the web being conveyed.

Thus, it is seen that with either embodiment, the supporting chambersare pressurized to provide the desired force of the individual chamberhousings against the traveling belts. Then, the individual pressurechambers can be either pressurized with the same pressure or anotherpressure to provide the operating dewatering force against the belt,felt or web as well as supplying the lubricating water seeping out overthe peripheral edges of the chamber housings from the pressure chambers.The embodiments wherein support and pressure chambers are separatelypressurized are, of course, more flexible than the embodiments whereincorresponding support and chamber housings are linked with a hydraulicconduit to provide the same pressure within each support and pressurechamber.

In operation, the pressure in the second pressure chambers 20b and 21bis higher than the first pressure within chambers 20a and 21a. The wallsbetween the chambers are sufficiently thick and strong to isolate thechambers from each other, but as the belt passes from a first chamber toa succeeding chamber, only a limited pressure drop for a brief instantof time is felt by the web when the pressure increases stepwise as shownin the graph of FIG. 2. The pressure in the third belt pressure chambers20c and 21c is higher than in the second belt pressure chambers 20b and21b.

This is shown in FIG. 2 where the pressure in the first portion of thepressing zone is shown at 31a, the pressure in the second portion at31b, and the pressure in the third portion at 31c of the pressuregraphline 31. The coordinates 32 and 33 indicate pressure and positionof the web as it passes through the pressing zone, respectively. As willbe noted, when the web passes the trailing edge of the last portion ofthe pressing zone, the pressure immediately and suddenly drops back tozero, so that web rewetting is kept at a minimum. By sudden drop inpressure on the web from maximum pressure, travel of the moisture fromthe felt back into the web, i.e. rewetting, will be minimized. Thetrailing guide rolls 13 and 15 are maintained at a position so thatthere is no contact between the paper and felt after they pass thepress.

It will be understood that the method and principles of this inventionmay be employed in other structures. For example, the lower belt may besupported on a rigid surface which permits the belt to slide or travelthereover with the hydraulic pressure being applied solely to one belt.One form of this structure which may be employed is where the lower beltis carried on the surface of a rotating cylinder, and the chambers forapplying successive steps of pressure to the upper belt are arrangedarcuately. Also, the web may be carried directly on a roll surface withthe lower belt omitted. Another contemplated arrangement will utilizetwo felts with the web sandwiched therebetween, one felt against each ofthe belts. The additional felt is shown in FIG. 1 as a dashed linedesignated F'. The belts may be configured on their surface facing thefelts so as to permit improved passage of water to the felt such as bybeing provided with pockets or longitudinal or herringbone or shapedsmall grooves sufficiently small to prevent marking of the web butadequate to aid in the flow of water into the felt. Water removal meansmay be provided on the offrunning side of the belts after they areseparated from the felt to clear any residual water which may pass intothe belt grooves.

With the structure and method shown, the resultant disadvantages whenthe pressure is applied at a very fast rate are avoided. This rapidapplication of pressure which can cause a disruption in the webformation or structure is commonly called crushing. It occursparticularly to heavily beaten stock or heavy basis weight webs. Theavoidance of crushing and the disrupting force which is caused by highlocal fluid velocity is avoided without the necessity of lowering thespeed of the machine, and the pressure differential between thedifferent portions of the zones of the structure of FIG. 1 and thelength of the zones can be constructed dependent in part upon the speedat which the machine is to be operated. While the pressure normally willincrease stepwise between the successive portions of the zone as shownin FIG. 2, a slightly less drastic change between zone portions can beobtained by shaping or angling the sills which engage the belt on theoffrunning and onrunning side of the chambers. By relieving the trailingend of the sill on the onrunning side, and perhaps elongating the sill,the rate of pressure increase from one portion of the pressing zone tothe next, will be more gradual.

I claim:
 1. A press structure for performing a dewatering operation inthe process of formation of a traveling fibrous web, comprising:at leastone felt for traveling with the web in water receiving contacttherewith; a pair of looped, fluid impervious belts for traveling withthe web for carrying the web and felt therebetween; first hydraulicpressure means for applying a uniform first pressure to a first areaalong the travel of the belts to force water from the web at a high rateof speed, said rate being less than that at which fiber crushing occurs;second successive hydraulic pressure means immediately adjacent thefirst hydraulic pressure means for applying a uniform second pressure toa second area along the travel of the belts, said second pressure beinggreater than the first pressure and sufficient to cause additional waterto flow from the web at a second rate less than the rate at which fibercrushing occurs so that the water can flow from the web without excesswater flow velocity building up within the web; said first and secondhydraulic pressure means each including a beam and a chamber housingarranged to define a hydraulic supporting chamber therebetween; eachchamber housing including a pressure chamber exposed to the travelingbelts, so that pressurized fluid supplied to the supporting chamberurges the chamber housing against the traveling web and pressurizedfluid applied to the pressure chamber provides water expressing forceagainst the web; and each beam and chamber housing being linked by aflexible diaphragm to define the peripheral walls of the supportingchamber.
 2. A press structure constructed in accordance with claim 1,further including:at least one guide rod to guide each chamber housingin movement normal to the belts under the impetus of operating forces.